It stands for "never give up, never surrender" and derives from the movie "Galaxy Quest", a Star Trek parody. It was the most famous quote of the commander Jason Nesmith, played from Tim Allen.

Nowadays it used as farewell in e-mails and short message conversations and should give the other person new hope and support if something bothers him or her.

Jason Nesmith: As long as there is injustice, whenever a Targathian baby cries out, wherever a distress signal sounds among the stars, we'll be there. This fine ship, this fine crew. Never give up... and never surrender.